Answering your question properly requires understanding the specific application for which you want to adhere silastic tubing to bone using an adhesive that contains cyanoacrylate.

Is the application to be performed inside the body during a surgical procedure? If so, you must account for blood and fluid at the bond site. In such cases, it is advisable to contact bone cement manufacturers such as Stryker Howmedica, Zimmer, Biomet, Becton Dickinson, 3M, or other similar manufacturers. Johnson & Johnson manufactures a skin-contact cyanoacrylate for treating wounds and cuts that is available at commercial drug stores as an over-the-counter option. For industrial applications, it is best to contact the manufacturer directly.

Is the bond application to be performed outside the body and then implanted for longer than 29 days? If so, you must consider the use of FDA-approved implant-grade materials. In such cases, I would recommend that you contact NuSil Silicone Technology or another manufacturer of implant-grade adhesives to inquire about a suitable implantable silicone material.

Is the application to be used in preparation for a demonstration model, a lab model, or another type of nonimplant application? If so, what is the application? In such cases, I would normally recommend 222/450 cP cyanoacrylate adhesive from Dymax, or I would advise you to contact Dow Corning, Momentive Performance Materials, NuSil Silicone Technology, Wacker, Rhodia, or Henkel for a one- or two-part silicone adhesive. In some cases, a surface preparation such as a plasma treatment will help adhere the silicone to the bone.

While cyanoacrylates stick well to bone, they do not stick well to inert silicone elastomers such as silastic. Do you require a cyanoacrylate-based adhesive? If so, I would usually recommend a one- or two-part silicone adhesive that will stick to both the silicone tubing and the bone. If you must use a cyanoacrylate-based adhesive, we highly recommend priming the silicone surface or preparing it with a plasma or corona treatment to improve adhesion.

As you continue to look for a suitable adhesive, I would recommend that you determine whether your silastic tubing is indeed Dow Corning’s Silastic-brand material or generic silicone tubing. Determining the type of silicone you have will be helpful, since some adhesives stick better to platinum-cured silicone tubing while others stick better to peroxide-cured silicone tubing—the two most common types of tubing on the market.

Conventional shape-memory nitinol only changes shape upon heating, not cooling. There have been recent reports of successful two-way shape-memory actuators, although the amount of recoverable strain is generally about 2%, which is much lower than the typical 6–8% achieved in one-way memory. Therefore, I recommend that the design be modified to make use of one-way shape recovery upon heating, with a biasing force acting against the shape-memory element to return it upon cooling. This type of two-way shape-memory device that uses the one-way shape memory effect acting against bias forces has demonstrated large strains, high forces in both the heating and cooling directions, and excellent long-term stability. The typical biasing force is a conventional spring that is stronger than the low temperature Martensite that returns the nitinol to its low-temperature shape, yet weak enough to deform when the nitinol is heated and transforms to the high-temperature Austenite phase.

Light-curing acrylate adhesives cure by exposing photoinitiators to certain wavelengths of light, which break apart the photoinitiators into radical species. These radical species react with oligomers to create long chains and cross-links. When a light is shined on the adhesive, it turns into a solid. By using a high-intensity light of >1 W/cm² across a broad spectrum range of 300–450 nm, there is usually so much energy that the adhesives crosslink extremely rapidly, leaving a firm, tack-free surface. However, some monomers and oligomers, which are the building blocks of these adhesives, may occasionally be susceptible to oxygen inhibition during the cure process. If oxygen is present at the surface of an adhesive, it can penetrate into the adhesive and interfere with radical polymerization, leaving unreacted monomers and oligomers at the surface. These tacky substances on the surface of the adhesive  can leave traces of a wet residue on a gloved hand.

Many newer adhesives are designed to be tack free under medium- and high-intensity conditions and when using the proper light wavelength. Generally, higher light energies, or lower wavelengths (200–300 nm) result in a better surface cure, but with such energy and light conditions, it is necessary to limit the depth of cure. Lower light energies, or higher wavelength (400–500 nm), result in greater depth of cure but may cause oxygen inhibition. The UV/visible-light spectrum in the 300–450-nm range seems to result in the best blend of surface cure and depth of cure.

Another consideration is lamp intensity. Higher-intensity lamp systems can put out massive amounts of intensity, with some spot lamp systems emitting up to 15–20 W/cm2 (measured at 365 nm). It is easy to cure most adhesives using such high-intensity lamps, which emit both ultraviolet (UV) and visible light.

Most adhesives have a minimum intensity threshold at which they become tack free within a specific period of time and with a minimum total energy threshold to achieve full cure. Consider the equation

J/cm² = W/cm² × sec

The intensity in Watts can be varied against the time of exposure in seconds to achieve the same amount of Joules per square centimeter. For example, 2 J/cm²= 2 W/cm² × 1 sec, or 2 J/cm² = 0.02 W/cm² × 100 sec. While it may appear that the same amount of total energy results in both situations (high intensity for a short time or low intensity for a long time), the minimum intensity threshold may show that a tack-free state cannot be attained if a minimum power level of 0.1 W/cm² is not maintained.

Some people find it beneficial to tackle the tack problem in a different way: by flooding the cure area with nitrogen or argon gas. Since nitrogen molecules are large, they cannot penetrate into the surface of the adhesive easily, providing a tack-free surface even when low-intensity light sources are used. This is helpful in situations in which the substrate is sensitive to heat or UV light.

The choice of adhesives is also important. Laboratory advances are enabling chemists to create adhesives that become tack free at lower intensities. Many new adhesives, with different properties, are used in potting and coating applications. Previously, only high-durometer materials (D60–80) became tack free. Now chemists have found ways to get soft yet tack-free adhesives with low-durometer ranges in the A40–60 range.

With the proper adhesive, process, light wavelength, time, and lamp intensity, a tack-free surface should be attainable.

The best adhesion is generally achieved with a bond gap of 0.002 to 0.006 in. If the bond line is too thin, the adhesive can squeeze out of the bond line, creating a void or an air bubble in the bond line. If the bond line is too thick, you end up relying on the inherent (tensile) strength of the adhesive to hold itself together. Cohesive failure is more likely to occur with some softer adhesives if the bond line is too thick.

There are many ways to remove the oxide from nitinol, including acid etching, mechanical polishing, grinding, and electropolishing. Acid etching leaves the surface rough and mottled, which is good for adhering coatings. Mechanical polishing, which is specific to wire, leaves the surface with a bright, smooth,  sanded appearance. This type of polishing is good when the wire needs to look similar to stainless steel. Centerless grinding is capable of holding very tight tolerances on discrete lengths of wire or tube. Electropolishing is typically used when nitinol will be implanted and is done as the last step in the process. This process creates a mirror-smooth surface and creates a biostable oxide. Some suppliers of nitinol have developed proprietary processes to remove the oxides that do not require the use of harsh acids but yield similar results.

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